She was born on 23 December 1922 at Pesch (Germany), the eldest of the two daughters of Prince and Duke Robert Prosper von Arenberg and of his wife, née Princess Gabrielle von Wrede. Her younger sister was Anna-Eugenie, who in 1952 married to Archduke Felix of Austria (while a maternal cousin of them, Princess Anna-Gabriele von Wrede, married to Archduke Rudolph of Austria, Felix's youngest brother).
In 1943 she married to Baron Karl Theodor von und zu Guttenberg (1921-1972), who later became a noted CSU politician. They became parents of five children: Elisabeth (Countess Franz Ludwig von Stauffenberg), Georg-Enoch (a conductor and owner of the winery estate Reichsrat von Buhl), Michaela (Baroness Johannes Heereman von Zuydtwyck), Benedikta (who died a few weeks after her birth) and Praxedis (Baroness Albrecht von Boeselager).
Her husband died of ASL in 1972.

She is survived by four of her children, her children-in-law, 19 grandchildren (among whom is Baron Karl Theodor von und zu Guttenberg, a German CSU politician and former federal government minister) and 20 great-grandchildren.

Noblesse & Royautes has some information about Princess Laetitia d'Arenberg. They also shared a link to a documentary which shows Princess Laetitia talking about her childhood in Europe, where they lived with the Italian Royal Family whilst they (the Italian Royals) were in exile. They also lived near the Count and Countess of Barcelona, the parents of King Juan Carlos:

__________________"She is a little angel and like her name, she brings sunshine even on cloudy days. From the bottom of our hearts, we would like to thank each and every one of you for your lovely best wishes for our daughter. She feels very loved". HRH Princess Madeleine, Duchess of Hälsingland and Gästrikland on her daughter, HRH Princess Leonore, Duchess of Gotland.